Improve bit perturbation in TupleHashTableHash.

The changes in b81b5a96f4 did not fully
address the issue, because the bit-mixing of the IV into the final
hash-key didn't prevent clustering in the input-data survive in the
output data.

This didn't cause a lot of problems because of the additional growth
conditions added d4c62a6b62. But as we
want to rein those in due to explosive growth in some edges, this
needs to be fixed.

Author: Andres Freund
Discussion: https://postgr.es/m/20171127185700.1470.20362@wrigleys.postgresql.org
Backpatch: 10, where simplehash was introduced

src/test/regress/expected/groupingsets.out

@@ -1183,29 +1183,33 @@ explain (costs off)
 -- simple rescan tests
 select a, b, sum(v.x)
   from (values (1),(2)) v(x), gstest_data(v.x)
- group by grouping sets (a,b);
+ group by grouping sets (a,b)
+ order by 1, 2, 3;
  a | b | sum 
 ---+---+-----
- 2 |   |   6
  1 |   |   3
+ 2 |   |   6
+   | 1 |   3
    | 2 |   3
    | 3 |   3
-   | 1 |   3
 (5 rows)
 
 explain (costs off)
   select a, b, sum(v.x)
     from (values (1),(2)) v(x), gstest_data(v.x)
-   group by grouping sets (a,b);
-                QUERY PLAN                
-------------------------------------------
- HashAggregate
-   Hash Key: gstest_data.a
-   Hash Key: gstest_data.b
-   ->  Nested Loop
-         ->  Values Scan on "*VALUES*"
-         ->  Function Scan on gstest_data
-(6 rows)
+   group by grouping sets (a,b)
+   order by 3, 1, 2;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ Sort
+   Sort Key: (sum("*VALUES*".column1)), gstest_data.a, gstest_data.b
+   ->  HashAggregate
+         Hash Key: gstest_data.a
+         Hash Key: gstest_data.b
+         ->  Nested Loop
+               ->  Values Scan on "*VALUES*"
+               ->  Function Scan on gstest_data
+(8 rows)
 
 select *
   from (values (1),(2)) v(x),

src/test/regress/sql/groupingsets.sql

@@ -342,12 +342,13 @@ explain (costs off)
 
 select a, b, sum(v.x)
   from (values (1),(2)) v(x), gstest_data(v.x)
- group by grouping sets (a,b);
+ group by grouping sets (a,b)
+ order by 1, 2, 3;
 explain (costs off)
   select a, b, sum(v.x)
     from (values (1),(2)) v(x), gstest_data(v.x)
-   group by grouping sets (a,b);
-
+   group by grouping sets (a,b)
+   order by 3, 1, 2;
 select *
   from (values (1),(2)) v(x),
        lateral (select a, b, sum(v.x) from gstest_data(v.x) group by grouping sets (a,b)) s;